Yes, you can catch-crop anywhere there's a bare bit of soil; use jerry's list. Catch cropping is when you sow a quick maturing crop in this vacant gap. Suitable crops for filling such gaps include spring onions, radishes and lettuce - they mature quickly and can be 'out of the way' quickly, and I don't believe they throw out rotations. You can also grow fast growing crops in ground that is not planted until the summer. For example, you can sow carrots or radishes in the early spring, and then harvest before planting late-sown cabbages on the same ground in late spring/early summer.
If you enjoy pea shoots (and who doesn't ;D) you can plant a small handful of peas in a gap, or a short row, and enjoy them most months of the year, because they don't need to mature to be eaten. And they will enrich the soil.
